Car bottom type furnaces are used in many industrial applications for heat treating metal parts. One long established use is in connection with heat treat furnaces used in forging operations. Accordingly the invention will be described in conjunction with this use.
During the processing of forged steel parts from ingot to finished product, the semi-finished product must be subjected one or more times to heat treatment to attain certain desirable properties such as ductility, hardenability, etc. The partly or semi-finished product is conventionally placed by a crane or other means onto a car bottom which runs on rails, and the car bottom with the parts to be heat treated are rolled into a furnace structure having a top, two side walls, and two end walls, at least one of which end walls can be lowered and raised to enable the car bottom to be rolled in and out. The wheeled, movable car bottom forms the bottom of the furnace after it is wheeled into position and the end entry wall closed.
After subjection to hot gases in the furnace for a predetermined time the end wall or door is opened and the car bottom with the treated parts is rolled out and moved to the next processing station.
It is essential that the furnace be as tight as operating conditions permit and have a long useful life between relinings for economic reasons. In this connection the seal which must be formed between the lower edges of the side and end walls, and the car bottom, must be tight and efficient so that loss of the high temperature gases inside the furnace is minimized to the maximum practicable extent, and infiltration of relatively cold ambient air, which would lower the efficiency of the furnace, is precluded. Further, since the temperature in the furnace may be as high as 2400 degrees F, the furnace elements become very hot and radiant heat loss can be substantial if the sealing system is not efficient and cracks are present.
A particular problem has been the maintenance of the castings which confine the car top refractory. As is well known, the car is usually constructed in the form of a flat bed structural steel frame assembly which is supported by two or more axle and wheel assemblies, and a layer of ceramic brick which forms a bottom surface upon which the parts to be treated rest during their dwell in the furnace. In many current structures the upper edges at least of these castings are exposed to the heat of the furnace and, unless constructed of very expensive, exotic alloy materials, oxidize and lose their strength before the end of a furnace campaign.
Conventional methods of sealing include sand seals, water seals, barometric seals, fiber swing seals and fiber abrasive seals. Each of these conventional systems has one or more drawbacks which this invention overcomes.
Sand seals, though they can be quite effective, are subject to loss of sand and hence loss of efficiency, require high alloy, expensive castings to contain the sand, and are rather difficult to effectively seal at the corners.
Water seals, which can be quite effective, are subject to evaporation and hence loss of effectiveness (which experience proves to be a common problem even in the most maintenance conscientious shop), are prone to refractory damage due to contact between the refractories and water, and, as a system, are quite costly.
Barometric seals are subject to radiation loss and infiltration and exfiltration of cold ambient air and hot combustion gases respectively.
Fiber abrasive seals require substantial maintenance and replacement costs are quite high.
Fiber swing seals require high alloy castings which can prove uneconomical over time. Such seals are now probably the state of the art in sealing at least low temperature car type furnaces. Usually ceramic fiber is pushed into the gap between the car and the furnace. However, the problem with swing type fiber seals is that castings are required to restrain the car top refractory, and with such a swing type seal the castings are subjected to furnace operating temperatures. At forging temperatures which, as mentioned, can be up to 2400 degrees F, these castings oxidize and lose their strength. The only way to ensure continuous and relatively maintenance free use of a swing seal at high temperatures is to construct the components of exotic alloy castings. These however are very expensive.